Hey guys,

I have to create a firework using particle systems in DirectX 9 but the math/physics stuff is killing me. I've been fiddling around with this for hours and couldn't get it to work as I want it to, which is why I'm now asking here.

I have a projectile flying physically correct that will burst into smaller particles after some time. I want the particles to be dispersed in a cone-like shape defined by the direction of the original projectile and an angle alpha, that are both known. I tried to make a little sketch of what I want to achieve but if you need clarification, feel free to ask.

[attachment=19401:Projectile.png]

I have to determine the velocity/flying direction for the particles. As I said, I know the original projectile's velocity/direction and the angle of the cone but I don't know how to combine these two to achieve what I want.

Please help me. Thank you very much in advance.

subparticle_velocity.x = cos(alpha) * projectile_velocity.x - sin(alpha) * projectile_velocity.y;
subparticle_velocity.y = sin(alpha) * projectile_velocity.x + cos(alpha) * projectile_velocity.y;

 subparticle_velocity = projectile_velocity * rotation

Thank you very much for your anwer, I will test it soon. Given a 3d space (including a z-component of the velocity) would it be possible to calculate the subparticle velocity in a similar fashion?